JNCL is a devastating progressive neurologic disorder. Initial symptoms for children with JNCL typically occur between ages 5 and 9 years and include progressive impairment of vision, seizures, difficulty with movement, and dementia. Scientists are still learning about how and why this disorder occurs. Although the seizures associated with JNCL can be treated, unfortunately there are no specific treatments for the underlying cause of JNCL at this time. Children with JNCL have antibodies to parts of brain cells. In animal models of JNCL, studies have shown that minimizing the ability to make antibodies may lead to slowing of the progression of the disease. Mycophenolate (CellCept) is a medication approved by the FDA for prevention of kidney transplant rejection in children; it acts by minimizing the formation of antibodies and therefore, is potentially an effective treatment for JNCL. A 22 week study of mycophenolate in children with JNCL is proposed. Each child will receive eight weeks of mycophenolate, and eight weeks of placebo, separated by four weeks off study medication. Investigators will be blinded to the order of treatments for each child (mycophenolate followed by placebo, or placebo followed by mycophenolate). Signs and symptoms of JNCL will be evaluated at the beginning and end of each eight week study period. Between evaluations, children will be monitored closely for potential medication side effects. The main objective is to assess whether patients with JNCL are able to complete a course of this medication without difficulty and also examine the impact of a short course of mycophenolate on signs of JNCL, including seizures, behavior, thinking, and motor function.